The present invention relates to the field of antennae for transmitting radiation, and more particularly, to a split monopole antenna for application in the triple frequencies of an emergency locator transmitter.
United States and Canadian law requires the use of emergency locating transmitters (ELTs) on all small aircraft traveling more than 25 miles from an airport and emergency position indicating radio beacons (EPIRBs) on certain classes of marine vessels. ELTs and EPIRBs are essentially the same device which transmit an audio tone on legislatively assigned frequencies of 121.5 MHz and 243 MHz indicating that a distress incident has occurred. The audio tone generated by these devices is provided by a distress modulation signal legislatively assigned to have a 2 to 4 Hz cyclic waveform wherein each cycle has a downward sweep of at least 700 Hz between 300 and 1600 Hz. The distress waveform demodulated in a conventional AM receiver provides a siren-like audio tone that is recognized by distress band observers. Search and rescue personnel, such as the Civil Air Patrol, search for the location of the distress transmission and initiate rescue operations. The distress transmission, however, contains no information to determine the identity/owner of the distress beacon. Knowledge of the identity/owner would, for example, enable the rescue coordinator to assign priorities and resources in a multiple emergency situation so that the emergencies that are critical from a time survival relationship are attended to early.
The introduction of a third emergency channel, the 406 MHz, by COSPAS-SARSAT, an international organization, during the early 1980s has corrected this system limitation. The 406 MHz emergency signal is a high energy pulse onto which owner/operator unique information is modulated. The 406 MHz has, in addition, improved frequency stability.
Satellite-aided search and rescue systems have been developed to augment existing search and rescue force capabilities to detect and locate ELT/EPIRB signal sources. Satellites aid the distress monitoring coverage by their high orbital altitude. The orbiting satellites respond to low level 121.5 HMz distress signals as well as high level 406 MHz data signals in a form specified by COSPAS-SARSAT and in the United States by the Federal Communications Commission. The 406 MHz information bursts contain information concerning user identification, country or origin and the category of the emergency beacon (e.g. maritime or aviation). The 406 MHz information is either processed on board the satellite or relayed to ground based instrumentation for processing. The continuous low level 121.5 MHz signal enables search and rescue personnel in close proximity to the emergency site to determine a final location to within a radius of approximately one kilometer.
Existing ELT/EPIRB products are mostly of the 121.5/243 MHz type. Other existing ELT/EPIRB radiate at either 406 MHz only, or at 121.5 and 406 MHz. Still other distress beacons with all three frequencies may be available within the marine sector (i.e. EPIRBs). The marine application tends not to place restrictive requirements on weight and size. Weight and size restrictions, however, are common for ELTs for use on aircraft. U.S. Pat. No. 3,653,053 to St. Vraine et al., for example, discloses a multi-frequency antenna, but is complex and its large size and high weight make it impractical to implement on a small xe2x80x9csurvivalxe2x80x9d ELT.
The present invention provides a split monopole antenna that provides for simultaneous transmission of 121.5, 243 and 406 MHz emergency signals using a simple, lightweight structure. The invention provides a design that allows the antenna to be stowed in an aircraft as part of the ELT in a compact manner during non-deployment and after deployment allows the ELT to float in water in an upright orientation. The present invention is meant to replace existing certified 121.5/243 MHz ELTs; the structural requirements of the present invention are, therefore, dictated by the symmetrical, small radius, lightweight and short length configuration of the simple monopole antenna of the prior art. Although the prior art mechanical restrictions dictate a similar structure, the electrical properties require that the present invention have significantly improved efficiency at 121.5 and 243 MHz over the prior art, and a highly efficient and well defined radiation pattern at 406 MHz
In one embodiment of the present invention, the monopole antenna comprises three radiating elements. A first radiating element is electrically coupled to a transmitting unit and radiates a 406.025 MHz signal; the second radiating element is electrically coupled to the first radiating element by way of a first band rejection filter and both elements radiate at 243 MFz; and the third radiating element is electrically coupled to the second radiating element by way of a second band rejection filter and all three elements radiate at 121.5 MHz. The effective length of each radiating elements at 121.5 and 243 MHz is approximately one quarter wavelength. The effective length of the radiating element at 406 MHz is slightly shorter than one quarter wavelength. Accordingly, each radiating group is physically shorter at progressive higher frequencies.
In an alternate embodiment of the invention, the monopole antenna comprises a first radiating element electrically coupled to a transmitting unit that radiates at 406 MHz and a single piece radiating rod that defines a second and third radiating element defined by the placement of a second band rejection filter. The first radiating element connects to the rod by way of a first band rejection filter.
It is therefore an object of this invention to provide a triple frequency ELT that will occupy the same fit and form as an existing certified 121.5/243 MHz ELT by survivors of downed aircraft.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.